Nowadays, along with communication speed-up of communication equipment and density growth of integrated circuits, there is a trend toward the increasing number of lead wires for electronics parts used for them. For example, there are conventional electronics parts having a lot of lead wires, such as QFP (Quad Flat Package), SOIC (Small Outline Integrated Circuit), etc.; however, in recent electronics parts having further multiple functions, such QFP and SOIC, etc. more number of lead wires is needed. Then, BGA (Ball Grid Array) becomes to be used as an electronics part having increased number of lead wires. There are many kinds of BGAs, such as PBGA (Plastic Ball Grid Array), TBGA (Tape Ball Grid Array), CBGA (Ceramic Ball Grid Array) etc. Especially, the CBGA is apt to be used for super computers and the like and it requires high reliability.
The CBGA is composed of a ceramic substrate and a printed circuit board (for example, glass epoxy substrate, etc.) and generates heat when a voltage is applied. In accordance with this heat generation, the ceramic substrate and the glass epoxy substrate are expanded. When the voltage applied to the CBGA is removed, the ceramic substrate and glass epoxy substrate are shrunken. As described hereinbefore, the ceramic substrate and glass epoxy substrate are repeatedly expanded and shrunken by applying the voltage to the CBGA and removing the voltage from it.
In general, a coefficient of thermal expansion for the ceramic substrate is 8 ppm/° C. and the coefficient of thermal expansion of the glass epoxy substrate is 15-20 ppm/° C. Therefore, a thermal expansion difference between the ceramic substrate and the glass epoxy substrate causes heat stress in the ceramic substrate and the glass epoxy substrate. Recently, a ceramic column grid array (hereinafter referred to as “CGA”) is used instead of the CBGA wherein the CGA is composed of a column having a better ability of absorbing the heat stress than that of a solder ball.
A general method for forming the CGA and mounting the CGA on the glass epoxy substrate will be discussed. First, solder paste is applied to an electrode portion of the ceramic substrate, which is not shown in a drawing. Then, a mounting jig is positioned on the ceramic substrate for mounting a column at a right angle with the electrode portion. The column is inserted into a through-hole made at the mounting jig so that the column stands within the solder paste on the electrode. They are put in a heating furnace, such as a reflow furnace, while keeping their relationship, and they are heated under predetermined temperature condition. Then, the solder paste applied to the electrode portion of the ceramic substrate is melted and the ceramic substrate is soldered with the column to form the CGA.
For mounting the CGA on the glass epoxy substrate, the solder paste is applied to the electrode portion of the glass epoxy substrate. Then, the column of the CGA is mounted on the electrode portion and is heated by the reflow furnace. The solder paste is melted, and the column and the glass epoxy substrate are soldered to each other to mount the CGA on the glass epoxy substrate. As described hereinbefore, before the CGA is mounted to the glass epoxy substrate, they are heated twice by the reflow furnace.
A high-temperature solder is frequently used for the CGA column in order to prevent the column from being melted in two heating steps by the reflow furnace or to prevent the column from being melted by heat that is generated by an IC chip mounted on a super computer and the like.
Further, since the column connects the ceramic substrate to the glass epoxy substrate, if the column is hard, the above-described heat stress may cause crack and fracture at joints between the ceramic substrate and the column, joints between the glass epoxy substrate and the column and the column itself. Therefore, soft material is used as the high-temperature solder for the column in order to absorb the heat stress, and such material are, for example, Pb based Pb—Sn solder, such as 95Pb-5Sn, 89.5PB-10.5Sn having low Vickers hardness.
With consideration for environment concern, the column excluding Pb, namely, lead-free type column is required. Intermetallic compound, such as Cu3Sn, Cu6Sn5, formed by reaction of Cu (copper), Sn (tin) and Bi—Sn solder containing Bi (bismuth) as a main ingredient and the like are known as a replacement of the Pb—Sn solder.
However, since the intermetallic compound, such as Cu3Sn, Cu6Sn5, etc., and Bi—Sn solder and the like are hard and brittle, when they are used as the column for joining the ceramic substrate and the glass epoxy substrate similarly to the CGA, the heat stress cannot be absorbed. In addition, as described hereinbefore, the crack and fracture facilitate at the joint between the ceramic substrate and the column, the joint between the glass epoxy substrate and the column and the column itself.
A copper column using Cu (copper) of good electrical and heat conductivity is known as the column material except for the solder. Patent Document 1 discloses an electrical pin in which Cu is plated for antioxidation but it is not used for the CGA. In this electrical pin, a plurality of pin members in which copper wires are plated with Sn or Fe are held in hardening resin, wherein one ends of the held pin members are plated with Cu and the other ends are mounted on the ceramic substrate. By etching the hardening resin or the Sn or Fe plating, the pin members stand on and are connected to the ceramic substrate.
Patent Document 2 discloses a pin that removes the residual stress by heating Cu but it is not used for the CGA. In this pin, one end of a copper wire is pressed to form a head portion and the copper wire having the formed head portion is heated at 600° C.-900° C. for five minutes. This heating step removes the residual stress due to the processing strain produced by the press work.